1. Field of the Invention.
The present invention relates generally to a vessel heater assembly and in particular to an external, electrical heater assembly for a steam table.
2. Description of the Prior Art.
Vessels with heaters for heating their fluid contents are well known in the art. For example, steam tables are commonly used for food service by restaurants, cafeterias and other institutions for maintaining food at elevated temperatures. A typical steam table includes a relatively large vessel for containing water and one or more trays suspended over the water and adapted for containing the heated food.
In some of the prior art steam tables, submersible heaters are mounted within the vessels and comprise hollow tubes which contain electrical resistance wires. However, a problem with this type of steam table is that the tubular heaters are generally inserted through the walls of the vessels below their normal water lines. Rubberized gaskets are often provided to seal the heater openings, but they are susceptible to deterioration and failure, particularly since they are exposed to heat and humidity when the steam tables are in use. Another disadvantage of this arrangement is that the internal placement of the tubular heaters contributes to the cost of the steam tables. An example of this type of steam table is shown in the Bailey U.S. Pat. No. 4,499,819, which shows a conventional heating element mounted within the vessel of a steamer for restaurant use.
To avoid some of the aforementioned problems, steam tables with externally-mounted heaters have also been devised. For example, the McMahan U.S. Pat. No. 4,084,080 shows a towel heater and dispenser with an electrical heat resistance element mounted below the bottom of a water vessel. Another externally-mounted heater is shown in the Abbate U.S. Pat. No. 4,123,969, which discloses a yeast raising baking appliance wherein steam is generated in a water vessel with a perforated top. The bottom of the water vessel rests on electrical heating elements which in turn rest on a pad of thermal insulation.
Heretofore, steam tables have also been provided with heaters adhesively secured directly to the undersides of their vessels for conductively heating the vessel and convectively heating the water contained therein. Silicone rubber blanket heaters comprising fine resistance wires vulcanized between thin sheets of silicone rubber have previously been employed for this purpose. However, the blanket heaters often distort the bottoms of the vessels and such distortion can cause separation between the bottoms of the vessels and the blanket heaters with resultant "hot spots". Such hot spots can rapidly deteriorate a blanket heater because when pockets of separation are formed, conductive heat removal is impaired and convective heat removal is often insufficient to protect the separated portion of the blanket heater from overheating. Hence, the separated pockets tend to rapidly overheat in relation to the rest of the blanket heater, which overheating will eventually destroy the silicone rubber or the resistance wires or both.
Conventional thermostatic controls which are provided on many such steam tables usually offer no protection against such overheating because they are actuated by the temperature of the water in the vessel rather than the temperature at isolated locations on the blanket heater. Thus, even though the water temperature may be within normal operating limits, "runaway" heat gain may be experienced in certain pockets where separation from the vessel bottom occurs.
The thermostatic temperature controls that are used on many steam tables also have some disadvantages. With such temperature controls, a typical heater operates at full power until the thermostatic switch opens at a predetermined upper limit temperature and no current is applied until the temperature falls to a predetermined lower limit, at which time the thermostatic switch closes and the cycle repeats. Such thermostatic control systems contribute to premature deterioration of the components which are subjected to frequent temperature changes over a relatively broad range.
Another disadvantage with thermostatically-controlled heaters is that they are often intentionally oversized to operate at higher temperatures than are normally required so that the vessel water temperatures can be relatively rapidly increased. The higher operating temperatures of such heaters and their frequent on-off cycling tend to be relatively inefficient from the standpoint of energy consumption. Furthermore, the thermostatic switches contribute significantly to the costs of the steam tables on which they are mounted. Some of the problems with thermostatic switch heat controls can be avoided by using solid state, non-cycling temperature controls, but these tend to be even more expensive than thermostatic controls.
Heretofore, there has not been a heater assembly suited for steam table use with the advantages and features of the present invention.